PROJECT SUMMARY The goal of this proposal is to assess dopaminergic modulation of an associative circuit within the dentate gyrus (DG) of the hippocampus. As the principal region of the hippocampus, the DG acts as a gate of incoming cortical information and plays a critical role in hippocampal learning and memory. As a result, DG dysfunction has been implicated in diseases including epilepsy, anxiety, and depression. The DG is recognized for its role in pattern separation, a process that enables distinction between two similar contexts or memories. In this process, the DG transforms similar cortical input patterns into distinct output patterns that can be read by the CA3 region. The two main principal cells of the DG are granule cells (GCs) and hilar mossy cells (MCs). Both cell types are excitatory. GCs receive cortical input and convey DG output to the CA3 region. Sparse firing of GCs is thought to underlie pattern separation, and MCs are positioned to shape this firing through an associative circuit, or excitatory loop, with GCs, termed the GC-MC-GC circuit. MCs project close to GC somas along the hippocampal axis and also mediate feed-forward inhibition onto GCs, affecting the excitatory/inhibitory balance of input to GCs. The Castillo Lab has recently discovered evidence supporting that activity-dependent changes in this circuit likely play a critical role in DG information processing. The Castillo Lab has demonstrated that MC-GC synapses undergo a novel form of LTP which enhances the E/I balance onto GCs as well as GC firing, thus enhancing DG output. Very little is known about the effect of neuromodulatory inputs on the dynamic properties of this circuit, but as is true throughout the brain, neuromodulators can affect information flow in circuits to shape their function in a context-dependent manner. Dopamine is a neuromodulator recognized for its role in modulating hippocampal circuits and hippocampal function. Evidence suggesting the presence of dopaminergic inputs and functional receptors in the DG supports that dopamine may shape the dynamic properties of the GC-MC-GC circuit and play a central role in DG-dependent learning. To study the role of endogenous dopamine in the GC-MC-GC circuit, electrophysiology recordings will be performed in acute mouse hippocampal slices to monitor excitability, transmission, and plasticity within the circuit during optogenetic stimulation of dopaminergic inputs. To test the role of dopamine in DG-dependent learning, dopamine receptors will be knocked out from mouse GCs and MCs using a viral injection strategy and these mice will be assessed in behavioral tests of novelty detection, pattern separation, and contextual fear learning. This work can help elucidate the cellular and molecular mechanisms of DG function and thus can provide a foundation for the prevention and treatment of DG- associated pathologies.